solution to 2.37

diff --git a/src/sicp/ex2_37.clj b/src/sicp/ex2_37.clj
new file mode 100644 (file)
index 0000000..0047ff3
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+++ b/src/sicp/ex2_37.clj
@@ -0,0 +1,50 @@
+(ns sicp.ex2_37
+  (:use [clojure.test]
+        [sicp [ch2-2 :only (accumulate)]
+              [ex2_36 :only (accumulate-n)]]))
+
+;; (define (dot-product v w)
+;;  (accumulate + 0 (map * v w)))
+(defn dot-product [v w]
+  (accumulate + 0 (map * v w)))
+
+(deftest test-dot-prod
+  (let [v '(1 2 3)
+        w '(10 20 30)]
+    (is [= (dot-product v w)
+           140])))
+
+;; (define (matrix-*-vector m v)
+;;   (map <??> m))
+(defn matrix-*-vector [m v]
+  (map (fn [r] (dot-product r v)) m))
+
+(deftest test-matrix*-vector
+  (let [m '((1 2 3) (4 5 6) (7 8 9))
+        v '(1 1 1)]
+    (is [= (matrix-*-vector m v)
+           '(6 15 24)])))
+
+;; (define (transpose mat)
+;;   (accumulate-n <??> <??> mat))
+(defn transpose [mat]
+  (accumulate-n cons nil mat))
+
+(deftest test-transpose
+  (let [s '((1 2 3) (4 5 6) (7 8 9))]
+    (is [= (transpose s)
+           '((1 4 7) (2 5 8) (3 6 9))])))
+
+;; (define (matrix-*-matrix m n)
+;;   (let ((cols (transpose n)))
+;;     (map <??> m)))
+(defn matrix-*-matrix [m n]
+  (let [cols (transpose n)]
+    (map (fn [v]
+           (matrix-*-vector cols v))
+         m)))
+
+(deftest test-matrix-*-matrix
+  (are [x y] [= x y]
+       (matrix-*-matrix '((8 9) (5 -1)) '((-2 3) (4 0))) '((20 24) (-14 15))
+       (matrix-*-matrix '((0 -1 2) (4 11 2)) '((3 -1) (1 2) (6 1))) '((11 0) (35 20))))
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